Ni/Ce co-doping δ-MnO2 nanosheets with oxygen vacancy for enhanced electrocatalytic oxygen evolution reaction

The design and manufacture of strongly engaged, low-cost, and resilient oxygen evolution reaction (OER) electrocatalysts is the most challenging task in electrochemical hydrolysis. Herein, Ce and Ni co-doped MnO₂ (NiCe/MnO₂) nanosheets (NSs) with oxygen vacancy (V_O) and abundant active sites have been prepared in one step employing a defect strategy. The co-doping of Ce/Ni on the one hand reduced the catalyst particle size and increased the specific surface area, which promoted the exposure of more active sites. On the other hand, heteroatom doping altered the species the crystalline surface, stimulating the formation of Vo and thus activating the catalyst performance simultaneously. The OER performance of NiCe/MnO₂ NSs was significantly enhanced over the pure δ-MnO₂, with an overpotential of 170 mV (10 mA cm^?2), which was verified by density functional theory. This work shows a straightforward and practical method for making non-precious metal electrocatalysts with high electrochemical hydrolysis performance.     

Development of shape-engineered α-MnO2 materials as bi-functional catalysts for oxygen evolution reaction and oxygen reduction reaction in alkaline medium

In this work, three kinds of α-MnO₂ nano shapes, namely, nano-wires, nano-tubes and nano-particles have been prepared with a fine control over α-crystallographic form by employing hydrothermal procedure. The materials have been thoroughly characterized by X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) spectrometry, field-emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. The MnO₂ nano shapes are used as a model system for examining the shape-influenced bi-functional electrocatalytic activity towards oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline medium. The bi-functional role has been investigated by cyclic voltammetry and linear sweep voltammetry with rotating ring disc electrode (RRDE) techniques. It is found that α-MnO₂ nano-wires possess enhanced electrocatalytic activity compared to other two shapes namely nano-tubes and nano-particles despite the nano-tubes having a much higher specific surface area. The insight of bi-functional electrocatalytic activity is analysed in terms of catalyst surface with the help of first principles density functional theory (DFT) calculations based on the fact of surface energies and adsorption of water on the surface for a facile reaction.     

Tuning of electrocatalytic activity of WO3–TiO2 nanocomposite electrode for alkaline hydrogen evolution reaction

The study reports the synthesis of mesoporous WO₃–TiO₂ nanocomposite with tuned particle size (~7 nm), pore diameter (~4.9 nm), specific surface area (S_BET = 129.112 m²/g) and pore volume (V_tot = 0.185 cm³/g) by an acid catalyzed peptization method, and its utilization for the development of stable catalytic electrode with enhanced activity towards alkaline hydrogen evolution reaction (HER). The SEM and AFM analyses confirm the formation of good quality composite electrodes with improved surface roughness through electroless deposition method. The developed WO₃–TiO₂ nanocomposite electrode exhibits low overpotential value of 120 mV with an exchange current density of 6.20 × 10^?5 mA/cm², and a low Tafel slope value of 98 mV/dec. Apart from the high HER performance, the developed WO₃–TiO₂ nanocomposite electrode exhibits competency with the state-of-the-art electrode materials for alkaline HER in industrial processes with sustained catalytic activity, tolerance behavior and long-term stability.     

Template confined construction of Fe–NiCoP/NiCoP/NF heterostructures for highly efficient electrocatalytic oxygen evolution reaction

Rational design of oxygen evolution reaction (OER) electrocatalysts with advance nanostructures and composition superiority is an urgent need to promote electrocatalytic property. In this research, we fabricate Fe–NiCoP/NiCoP/NF electrocatalyst for OER via the interfacial scaffolding strategy with Prussian-blue-analogue (PBA) followed by low-temperature phosphating. The cube-on-sheet multimetallic-TMPs-based nanoarchitecture of Fe–NiCoP/NiCoP/NF exhibits outstanding OER performance, which only requires the overpotential of 201 mV to achieve a current density of 10 mA cm⁻² and possesses good durability up to 50 h in 1.0 M KOH solution. The superior OER property of Fe–NiCoP/NiCoP/NF is mainly characteristic to the rich composition that optimizes the electronic structure and the cube-on-sheet multimetallic-TMPs-based nanoarchitecture which can facilitate more effective active sites exposure and ultimately promote charge transfer at the same time. This research provides a new strategy for the construction of advanced nanoarrays structure and the improvement of the electrocatalytic performance of polymetallic phosphides, which offers its promising applications especially in energy storage and conversion technology.     

Tuning of electrocatalytic activity of Mn–O–Co composite for alkaline hydrogen evolution reaction

We report the enhancement in electrocatalytic activity of Mn–O–Co composite electrode developed through chemical reduction method. The Mn–O–Co composite electrode exhibits high catalytic activity with a low Tafel slope of 123 mV dec⁻¹ and a low overpotential of 117 mV at a current density of 10 mA cm⁻². The enhancement in electrocatalytic activity of Mn–O–Co composite electrode is due to the synergistic activity of MnO and CoO with the NiP matrix. The intermetallic interaction among the half-filled orbitals of manganese with the fully occupied orbitals of cobalt and nickel leads to an effective electron delocalization in the catalytic system which enhances the HER performance of the coating. The C_dl value of the composite electrode is in the order of 254 μF, which is approximately ten fold higher than the bare NiP coating, due to the enhancement in interaction between the Mn–O–Co composite electrode and the reactive species in the HER medium. The Mn–O–Co composite electrode shows promising characteristics as an electrocatalyst with long term stability and remarkable competency with the commercially available electrodes.     

Enhanced electrocatalytic performances of α-Fe2O3 pseudo-nanocubes for oxygen reduction reaction in alkaline solution with conductive coating

Exploring of high-efficient, low-cost and eco-friendly catalysts for oxygen reduction reaction (ORR) is one of the vital issues for fuel cells and metal-air batteries. Herein, α-Fe₂O₃ pseudo-nanocubes have been synthesized with a facile solvothermal method, and then α-Fe₂O₃@NC composite catalysts were prepared through chemical polymerization of pyrrole on α-Fe₂O₃, following with pyrolyzed in nitrogen atmosphere. The synthesized composite catalysts display enhanced ORR catalytic performances, including the positive shifting of the onset potential, improving of the limited current density with long-term stability compared to those of the pristine α-Fe₂O₃. The enhanced electrocatalytic performances could be ascribed to the low intrinsic and charge transfer resistances, the high content of pyridinic-N and/or FeN, the emergence of graphitic-N and abundant oxygen vacancy on the composite surface. This study here implies that the catalytic activity and stability of metal oxides with poor conductivity could be controlled and improved by simply coating with a nanoscale conductive layer, which shows promising potential applications as precious-metal free catalysts for various metal-air batteries and fuel cells.     

Crystal facet-dependent activity of α-Mn2O3 for oxygen reduction and oxygen evolution reactions

Electrocatalysts with different morphologies and specific exposed facets usually exhibit distinguished activities. In this work, two α-Mn₂O₃ with different morphologies and crystal facets were successfully prepared by solvothermal method and investigated as ORR/OER bifunctional catalysts for the first time. The results showed that the catalytic performance is affected by the crystal facet of α-Mn₂O₃, and the α-Mn₂O₃-cubic has better bifunctional activity than α-Mn₂O₃-octahedra. As revealed by catalyst characterization, the enhanced activity is originated with the nature of the exposed α-Mn₂O₃ (100) facet. The (100) facet with abundant low-coordinated surface oxygen sites makes the formation of more oxygen vacancies, which can improve the charge transfer and optimize the adsorption energies for intermediates, thereby enhancing the bifunctional activity for ORR/OER. This work highlights the correlation between OER/ORR activities and exposed crystal planes of α-Mn₂O₃ catalysts and it may deepen the understanding of facetdependent activity of α-Mn₂O₃ and points out a strategy to improve their catalytic activity by crystal facet engineering.     

Experimental and theoretical realization of an advanced bifunctional 2D δ-MnO2 electrode for supercapacitor and oxygen evolution reaction via defect engineering

Modulating the intrinsic physicochemical properties of crystalline 2D materials by dint of defect engineering largely enables multi-functionality. Uniform thin layered nanosheets further self-assembled at micro scale forming embossing structures of δ-MnO₂ were fabricated by microwave irradiation technique. The irradiation of UV/O₃ impacts incorporation of oxygen vacancy into the pristine system. Furthermore, detailed structural, morphological, surface analytical and electrochemical investigations evidenced outstanding energy storage and conversion activities. The asymmetric device δ-MnO₂-UVT//F-MWCNT with an extended potential window 1.5 V, exhibited maximum energy density of 39 Wh/kg at a power density of 468.75 W/kg. The defect structural design exhibited excellent electrocatalytic OER activity with lowest overpotential (η₂₀, 300 mV) and Tafel slope (71 mV/dec). The efficiency and stability of the illuminated material showed outstanding performances. To support our experimental findings, we have presented the electronic structures and quantum capacitance for pristine δ-MnO₂ and δ-MnO₂ with O vacancy employing Density Functional Theory (DFT) simulations. Presence of O vacancy makes a semi-conducting to metallic transition. The oxygen vacancies delocalize the neighboring electrons around the low coordinated Mn atoms and these delocalized electrons can be easily moved into the conduction band resulting improved conductivity in the material. In addition, the computed quantum capacitance tendency is as follows, δ-MnO₂-UVT > δ-MnO₂ which associates with the experimental supercapacitance behaviour of these systems.     

The structure-activity relationship of Pd–Co/C electrocatalysts for oxygen reduction reaction

The Pd–Co/C alloy catalysts with an atomic ratio of 3:1 were deposited at various pH values and reduced at different temperatures for oxygen reduction reaction (ORR). The structure-activity relationship of the prepared catalysts has been elucidated. The pH values and reduction temperatures during the preparation process affect the deposition and reduction rates of Pd and Co ions significantly, and thus the degrees of alloying, surface species, and ORR activities of the Pd–Co/C catalysts are also influenced. Due to the enhancement of Co surface segregation and the formation of Co oxide on the surface, a deterioration of ORR activity for the catalysts reduced at high temperatures and high pH values is observed. The catalysts deposited at pH value of 9 and reduced at a very low temperature of 390 K have well-formed Pd–Co alloy structure, Pd-rich surface, and excellent ORR activity.     

MgO as promoter for electrocatalytic activities of Co3O4–MgO composite via abundant oxygen vacancies and Co2+ ions towards oxygen evolution reaction

Oxygen evolution reaction (OER) is known as bottleneck problem during the water splitting process due to high energy barrier and non-availability of efficient nonprecious electrocatalysts. The cobalt oxide (Co₃O₄) in the spinel phase has limited OER activity and stability in the alkaline media. For this purpose, we have carried out the synthesis of Co₃O₄–MgO (CM) composite by wet chemical method and it offers abundant oxygen vacancies and Co²⁺ concentration for the efficient OER reaction. The effect of different amounts of MgO on the OER activity of Co₃O₄ was also studied. Despite inactivity of MgO towards OER, it creates high density of oxygen vacancies and favored the formation Co²⁺ ions at the surface, thus accelerated the OER kinetics. The physical studies were performed to investigate the morphology, crystalline structure, surface information and chemical composition using several analytical techniques. The optimized CM-0.1 composite produced an overpotential of 274 mV at 10 mAcm⁻² which is lower in value than the pristine Co₃O₄. The significant enhancement in the OER activity was verified by the large value of electrochemical active surface area values 12.8 μFcm⁻² and the low charge transfer resistance of 45.96 Ω for the optimized CM-0.1 composite. The use of abundance materials for the synthesis of CM composite revealed an enhanced OER performance, suggesting the dynamic role of MgO, therefore it could be used for improving the electrochemical properties of extended range of metal oxides for specific application especially energy conversion and storage devices.     

SrTi0.1CoxFe0.9-xO3-δ Perovskites for enhanced oxygen evolution reaction activity

We developed a series of Fe doping in Co-based perovskites SrTi_0.1Co_xFe_0.9-xO_3-δ (x = 0.5, 0.6, 0.7, 0.9) to investigate their OER activity and stability in alkaline media. Among all the samples, SrTi_0·1Co_0·5Fe_0·4O_3-δ (donated as STCF-154) shows wonderful OER activity with an overpotential of 0.37 V, a current density of 33.65 mA cm⁻² at 1.71 V, and a Tafel slope of 94.82 mV dec⁻¹. Besides, the potential of STCF-154 remained nearly unchanged for at least 8 h at a fixed current density of 10 mA cm⁻²_disk on GC electrode. The improved activity and stability are likely originating from the highly oxidative oxygen species O₂²⁻/O⁻ formed in STCf-154, which can easily migrate from bulk STCF-154 and “spillover” to the surface of the catalyst during OER process. The Fe doping in Co-based perovskites had synergetically enhanced activity and can be considered as a good candidate for the OER in alkaline solution.     

Enhanced hydrogen evolution reaction of WS2–CoS2 heterostructure by synergistic effect

Transition metal dichalcogenides (TMDs) have attracted significant research interest due to its promising performance in hydrogen evolution reaction (HER). Synergistic effect between materials interface can improve the electrocatalytic properties. In this work, the WS₂–CoS₂ heterostructure supported on carbon paper (CP) was elaborately fabricated by a three-step method. Owing to the synergistic effect, WS₂–CoS₂ heterostructure exhibits an excellent electrocatalytic activity with a low overpotential of 245 mV at 100 mA/cm² and a small Tafel slope of 270 mV/dec toward HER. We demonstrate that the increased specific surface area and conductivity of the heterostructure play a key role in enhancing the overall catalytic efficiency. Moreover, the crystal lattice distortion in the heterostructure could induce charge redistribution and improve electron transfer efficiency, which may also benefit the whole HER activity.     

IrO2/Nb–TiO2 electrocatalyst for oxygen evolution reaction in acidic medium

A corrosion-resistant Nb_0.05Ti_0.95O₂ material with high surface area was prepared by a sol–gel process. IrO₂ nanoparticles (about 16–33 wt%) were successfully loaded on Nb_0.05Ti_0.95O₂ powders as the electrocatalyst for oxygen evolution reaction (OER) in acidic medium. The IrO₂/Nb_0.05Ti_0.95O₂ catalyst with the IrO₂ loading of 26 wt% exhibits the best mass normalized cyclic voltammetry charge and mass normalized activity among all the IrO₂/Nb_0.05Ti_0.95O₂ catalysts because IrO₂ nanoparticles were uniformly supported on the surface of Nb_0.05Ti_0.95O₂ providing conductive channels to reduce the grain boundary resistance. Due to the anchoring effect of carrier on the catalyst, the stability of the supported IrO₂ was significantly improved as compared to the unsupported one. The IrO₂/Nb_0.05Ti_0.95O₂ catalyst with 26 wt% IrO₂ loading demonstrates the best effectiveness of the OER activity and cost.     

Interface engineering and heterometal doping Co–Mo/FeS for oxygen evolution reaction

The sluggish kinetics of the oxygen evolution reaction (OER) limits the development of water electrolysis technology and the long-term efficiency of hydrogen energy production. In addition, it is important to evaluate the reconstruction performance of OER catalysts for actual water electrolysis. We created a self-supported electrode with FeS film coated Fe foam as a substrate, ordered resoluble molybdate (MoO₄²⁻) anions in interlayers, and Co-doped as a catalytically active phase for the OER. The catalyst is capable of electrochemical self-reconstruction (ECSR). With the dissolution of molybdate and sulfur ions, the catalyst surface cobalt iron oxide (CoFe₂O₄) forms an active amorphous FeCoOOH, which is favorable for alkaline OER. We realized the introduction of new active sites in the catalyst reconstruction process. Finally, the composite CoFeO_x catalyst increased the specific surface area, promoted bubble transport, and enhanced electron mass transfer. The synergistic coupling effect of the catalyst makes it have excellent OER activity and stability. Remarkably, Co–Mo/FeS nanosheets afforded an electrocatalytic OER with a current density of 100 mA cm⁻² at a low overpotential of 321 mV. These discoveries open up new opportunities for the application of doping and template-directed surface reconfiguration, which holds promise as an effective electrocatalyst for the OER.     

Two step synthesis of TiO2–Co3O4 composite for efficient oxygen evolution reaction

For an active hydrogen gas generation through water dissociation, the sluggish oxygen evolution reaction (OER) kinetics due to large overpotential is a main hindrance. Herein, a simple approach is used to produce composite material based on TiO₂/Co₃O₄ for efficient OER and overpotential is linearly reduced with increasing amount of TiO₂. The scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) investigations reveal the wire like morphology of composite materials, formed by the self-assembly of nanoparticles. The titania nanoparticles were homogenously distributed on the larger Co₃O₄ nanoparticles. The powder x-ray diffraction revealed a tetragonal phase of TiO₂ and the cubic phase of Co₃O₄ in the composite materials. Composite samples with increasing TiO₂ content were obtained (18%, 33%, 41% and 65% wt.). Among the composites, cobalt oxide-titanium oxide with the highest TiO₂ content (CT-20) possesses the lowest overpotential for OER with a Tafel slope of 60 mV dec^?1 and an exchange current density of 2.98 × 10^?3A/cm². The CT-20 is highly durable for 45 h at different current densities of 10, 20 and 30 mA/cm². Electrochemical impedance spectroscopy (EIS) confirmed the fast charge transport for the CT-20 sample, which potentially accelerated the OER kinetics. These results based on a two-step methodology for the synthesis of TiO₂/Co₃O₄ material can be useful and interesting for various energy storage and energy conversion systems.     

Structure engineering of amorphous P–CoS hollow electrocatalysts for promoted oxygen evolution reaction

Oxygen evolution reaction (OER) is a key process involved in many energy-related conversion systems. An ideal OER electrocatalyst should possess rich active sites and optimal binding strength with oxygen-containing intermediates. Although numerous endeavors have been devoted to the modification and optimization of transition-metal-based OER electrocatalysts, they are still operated with sluggish kinetics. Herein, an ion-exchange approach is proposed to realize the structure engineering of amorphous P–CoS hollow nanomaterials by utilizing the ZIF-67 nanocubes as the precursors. The precise structure control of the amorphous hollow nanostructure contributes to the large exposure of surface active sites. Moreover, the introduction of phosphorus greatly modifies the electronic structure of CoS₂, which is thus favorable for optimizing the binding energies of oxygenated species. Furthermore, the incorporation of phosphorus may also induce the formation of surface defects to regulate the local electronic structure and surface environment. As a result of this, such P–CoS hollow nanocatalysts display remarkable electrocatalytic activity and durability towards OER, which require an overpotential of 283 mV to afford a current density of 10 mA cm^?2, outperforming commercial RuO₂ catalyst.     

Lead cathodes functionalized with magnetite particles with enhanced electrocatalytic activity for hydrogen evolution reaction in sulfuric acid solutions

The generation of molecular hydrogen through the electrochemical water splitting process has been extensively studied, due to the potential application of hydrogen to produce green energy with fuel cells. Researchers have been focused in the synthesis of electrocatalysts for the hydrogen evolution. The accumulative roll bonding (ARB) is a promising method which can process a large quantity of an electrode for industrial demand. In this research the ARB process is employed to synthesize lead base electrodes functionalized with magnetite particles (Fe₃O₄) to evaluate their electrocatalytic properties on the hydrogen evolution reaction (HER) in sulfuric acid solutions. SEM, EDS and FESEM techniques are used to characterize the functionalized lead cathodes. The effect of rolling passes, magnetite concentration, and suspended magnetite particles on the HER kinetics is studied using linear voltammetry, Tafel plots, and electrochemical impedance spectroscopy (EIS). The results revealed that lead cathodes functionalized with magnetite present great advantages over lead cathodes, such as: a decrease of 0.37 V in HER over-potential, an increase of 84.3 times in the exchange current density, a decrease in the charge transfer resistance of 98%. From a mechanistic viewpoint, HER is catalyzed by the presence of Fe²⁺ adsorbed on the lead cathode; ferrous ions are produced from the reductive dissolution of magnetite particles. Long-term electrolysis and multiple cyclic voltammetry tests (800 polarization cycles) revealed that the catalytic effect is maintained during 44 h of operation. According to the Tafel curves and EIS results, the Volmer reaction is the rate determining step of the HER on these functionalized cathodes.     

Electrocatalytic activity of Pt–Pd electrocatalysts for the oxygen reduction reaction in proton exchange membrane fuel cells: Effect of supports

Pt–Pd electrocatalysts supported on different types of support including domestic Hicon Black (HB), multi-walled carbon nanotubes (MWCNT) and titania (TiO₂) were prepared by a combined approach of impregnation and seeding, and compared to that prepared using the commercial Vulcan XC-72 (C). Their oxygen reduction reaction (ORR) activities in an acid electrolyte (0.5 M H₂SO₄) and in a single proton exchange membrane (PEM) fuel cell were evaluated. The type of support was found to affect the Pt–Pd electrocatalyst morphology and ORR activity. The Pt–Pd/C electrocatalyst had the smallest Pt particle size, better catalyst dispersion and a higher Pt:Pd M ratio compared to that of other types of supported Pt–Pd electrocatalysts. However, both in the acid solution and in a single PEM fuel cell, the ORR activities of the Pt–Pd/HB and Pt–Pd/CNT electrocatalysts were comparable to that of the Pt–Pd/C one. The ORR pathway of all supported Pt–Pd electrocatalysts were close to the four-electron pathway.     

High electrocatalytic effect of Au–Pd alloy nanoparticles electrodeposited on microwave assisted sol–gel-derived carbon ceramic electrode for hydrogen evolution reaction

Various Au–Pd bimetallic nanoparticles were electrodeposited on microwave irradiated carbon ceramic electrodes (MWCCE). Au:Pd molar ratios of 75:25, 50:50 and 25:75 were electrodeposited on MWCCE and their electrocatalytic activities for hydrogen evolution reaction (HER) were evaluated. Among them, the alloy with Au:Pd molar ratio of 25:75 showed highest electrocatalytic activity for HER. The structure and nature of these alloys were characterized by scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, inductively coupled plasma, and cyclic voltammetry. Alloying degree of bimetallic nanoparticles and electrodeposition time were optimized. The electrocatalytic activity of bimetallic nanoparticles was also compared with individual non-alloyed Au and Pd catalysts and the results showed that alloy nanoparticles have higher electrocatalytic activity for hydrogen evolution. The Tafel slopes ranges are obtained from 136 mV dec⁻¹ to 165 mV dec⁻¹ for HER on bare and modified MWCCE and kinetic parameters show that the Volmer step must control the HER. The stability of the best electrode is determined by chronopotentiometric and it showed a good stability.     

Electrodeposited amorphous nickel–iron phosphide and sulfide derived films for electrocatalytic oxygen evolution

The preparation of inexpensive and efficient electrocatalysts for oxygen evolution reaction (OER) is crucial in the widespread application of water electrolyzers. A simple one-step aqueous electrodeposition method is utilized to prepare amorphous nickel-iron sulfide (Ni–Fe–S) and phosphide (Ni–Fe–P) films on Ni foam. The deposited films are highly porous, and can convert to active electrocatalysts for OER. In 1 M KOH, the Ni–Fe–S shows the highest OER activity, and requires only 230 mV overpotential to reach 0.05 A cm^?2 OER current densities. The Fe–Ni–S also sustains the 30 h 0.05 A cm^?2 galvanostatic OER test. Ex-situ characterizations show that sulfur in the Fe–Ni–S is oxidized and leached into the solution during OER, and that (oxy)hydroxide layer is formed at the surface. The adsorption energy of the hydroxyl group, an OER intermediate, is tuned by the electron interaction between the Ni and Fe, and the Ni–Fe–S exhibits the optimum hydroxyl group adsorption energy and the most facile OER kinetics. Also, higher intrinsic OER activity is observed for the electrodeposited amorphous nickel phosphide-derived film than the amorphous nickel sulfide-derived film.